A plasma is a particular aggregate state which is produced from a gas. Each gas comprises in principle atoms and/or molecules. In a plasma, this gas is to a large degree ionized. This means that, by supplying energy, the atoms or molecules are split into positive and negative charge carriers, that is to say, into ions and electrons. A plasma is suitable for processing workpieces since the electrically charged particles are chemically highly reactive and can additionally be influenced by electrical fields. The charged particles can be accelerated by means of an electric field onto an object where they can release individual atoms therefrom upon impact. The released atoms can be conveyed away by means of gas flow (etching) or be deposited on other objects as a coating (production of thin films). Such a processing operation using a plasma is used in particular when extremely thin layers, in particular in the region of a few atom layers, are intended to be processed. Typical applications are semi-conductor technology (coating, etching, etc.), flat screens (similar to semi-conductor technology), solar cells (similar to semi-conductor technology), architectural glass coating (heat protection, anti-glare protection, etc.), storage media (CD, DVD, hard disks), decorative layers (colored glasses, etc.) and tool hardening. These applications have high demands in terms of precision and process stability. Furthermore, a plasma can also be used for the excitation of lasers, in particular gas lasers.
In order to generate a plasma from a gas, energy must be supplied to it. This can be carried out in different manners, for example, by means of light, heat, electrical energy. A plasma for processing workpieces is typically ignited in a plasma chamber and maintained. To this end, a noble gas, for example, argon, is directed into the plasma chamber at low pressure. By means of electrodes and/or antennae, the gas is subjected to an electrical field. A plasma is produced or is ignited when a plurality of conditions are fulfilled. Firstly, a small number of free charge carriers must be present, with the free electrons which are always present in a very small quantity being used in most cases. The free charge carriers are so powerfully accelerated by the electrical field that they release further electrons upon impact with atoms or molecules of the noble gas, whereby positively charged ions and additional negatively charged electrons are produced. The other free charge carriers are again accelerated and produce other ions and electrons upon impact. An avalanche effect is produced. The occurrences of discharge during the collision of these particles with the wall of the plasma chamber or other objects and the natural recombination counteract the constant production of ions and electrons, that is to say, electrons are attracted by ions and recombine to form electrically neutral atoms or molecules. Therefore, energy has to be supplied to an ignited plasma continuously in order to maintain it.
The energy supply can be carried out by means of a direct current (DC) power supply or an alternating current (AC) power supply. The frequencies which occur during plasma excitation with an AC power supply may be as high as in the gigahertz range.
In the plasma, brief and also longer-lasting flashes may occur, so-called arcs, which are undesirable. When such an arc is detected, it must be ensured that it is extinguished in the most rapid manner possible or does not fully develop.
In a plasma process, various signals can be used for arc detection and have corresponding indications for an arc, for example, the reflected power, the relationship of reflected power to forward power, current, voltage, phase angle, brightness of the plasma, optionally at one or more specific optical wavelengths, acoustic signals, DC bias which is produced, signals from an antenna in the plasma chamber, etc. Mathematical operations on these signals (for example, consideration of the slope by differentiating a signal, filtering) can increase the sensitivity.
The arc detection with fixed threshold values is often either not secure enough or too stringent. Furthermore, it is not flexible enough to react to changing operating parameters.